This invention relates to variable capacitance circuits in general and, in particular, to the use of such circuits in tuned resonant circuits such as voltage controlled oscillators (VCO's) with a programmable bias voltage.
Varactors are typically utilized in voltage controlled oscillator circuits. A varactor is essentially a reversed biased diode that exhibits a capacitance dependent upon the magnitude of the reverse bias voltage applied to it. When used in a resonant circuit, such as an oscillator, the frequency of resonance can be controlled by controlling the bias voltage applied across the varactor.
A typical application of VCO's are in frequency synthesizer circuits such as phase lock loops (PLL). In a PLL circuit, a voltage output of a phase detector is applied via a loop amplifier to the varactor of the VCO in order to control the VCO frequency. For example, the positive voltage applied to the varactor can be derived from the phase detector and loop amplifier while the low or negative side of the varactor is tied to ground or referenced to another fixed bias point. These polarities can be reversed if desired and the varactor's high side can be tied to a fixed bias point while the varactor's low side is tied to the loop amplifier.
When it is desired to provide a substantial range of frequency output of the VCO, the available control voltage applied to the VCO must vary sufficiently to allow the necessary change in capacitance of the varactor and consequently the VCO frequency. A significant problem arises when the available voltage range applied to the phase comparator is less than the required voltage range to change the capacitance of the varactor.
For example, a phase detector and loop amplifier operating on a five volt power supply may have an effective output range from 0.5 to 4 volts. This will allow only a variation of 3.5 volts in the voltage appearing across the varactor. While it is possible to increase the operating voltage applied to the phase detector and loop amplifier, this may be undesirable for a number of reasons.
In a portable radio device or the like, the circuit operating voltage is derived from batteries having a predetermined output. The voltage available to apply to the phase comparator, loop amplifier and varactor may be less than the battery voltage due to voltage drops across voltage regulators and the like.
While a DC to DC converter could be used to provide a step up operating voltage to the phase comparator and loop amplifier, such a circuit would introduce inefficiency in the operation of the device, as the phase comparator and loop amplifier would necessarily draw additional current and consequently consume more power than the circuits would when operated at a lower voltage.
In applications such as portable radios, conservation of battery energy is of upmost importance. Consequently, in applying increased operating bias voltage to the varactor circuit, it is desirable that increased voltages not be applied to other circuits that would increase battery current drain of the device. Such increased voltages might also be too high for the particular circuit (i.e. a low voltage circuit such as some CMOS IC processors).